Circuit board and method of manufacturing the same

ABSTRACT

A circuit board is manufactured by filling a via-hole formed in an insulating substrate with conductive material, disposing conductive layers on both sides of the insulating substrate, and forming alloy of component material of the conductive material with component material of the conductive layers. In the circuit board, therefore, the conductive material filled in the via-hole formed in the insulating substrate is securely connected electrically as well as mechanically to the conductive layers on both sides of the insulating substrate with high reliability.

FIELD OF THE INVENTION

[0001] The present invention relates to a circuit board including aninsulation substrate having a via-hole filled with conductive material,and conductive layers disposed to both sides of the insulationsubstrate. The invention also relates to a method of manufacturing theboard.

BACKGROUND OF THE INVENTION

[0002] According to the recent trend toward downsizing of electronicdevices, inexpensive circuit boards that are capable of mountingsemiconductor chips, such as LSIs, are strongly demanded for not onlyindustrial purposes but also for consumer products. It is imperativethat such circuit boards having multi-layered fine wiring patterns canbe manufactured easily with high yield rate and high reliability, forthe purpose of increasing packaging density.

[0003] A conventional circuit board includes a glass-epoxy board, whichis an insulation substrate made of woven glass cloth impregnated withepoxy resin, and copper foils bonded by heat-press or the like to bothsides of the substrate. In circuit board, patterns are formed byphoto-etching the copper foils, a through-hole is formed by drilling orthe like, and wiring layers between both side surfaces of the throughhole are then connected with copper plated on an interior wall of thethrough-hole.

[0004] In this method, an interior of the through hole, upon beingplated, lacks reliability because plating solution does not permeateeasily into the hole, and an area which is not plated thus tends to becreated, and this may cause a failure of electrical connection. This maycause in deficiency that a thickness of the plated copper deep insidethe through-hole, which can cause an electrical problem due to a largeresistance for an electric connection. It is difficult to mount acomponent to a portion where a through-hole is formed. And it isdifficult to plate a through-hole in a desired inner layer of amulti-layered substrate. These difficulties limit an arrangement ofwiring patterns and manufacturing processes of the circuit board, andalso hinder downsizing of the board.

DISCLOSURE OF THE INVENTION

[0005] A circuit board is manufactured through filling a via-hole formedin an insulating substrate with conductive material, forming conductivelayers on both sides of the insulating substrate, and forming alloy ofcomponent material in the conductive material and component material ofthe conductive layers.

[0006] In this circuit board, the conductive material in the via-holeformed in the insulating substrate is securely connected electrically aswell as mechanically to the conductive layers on both sides of theinsulating substrate reliably.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1A through FIG. 1D are sectional views of a circuit board,illustrating a method of manufacturing the board according to exemplaryembodiment 1 of the present invention.

[0008]FIG. 2A through FIG. 2D are sectional views of a circuit board,illustrating a method of manufacturing the board according to exemplaryembodiment 2 of the invention.

[0009]FIG. 3A through FIG. 3D are sectional views of a circuit board,illustrating a method of manufacturing the board according to exemplaryembodiment 3 of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0010] (Exemplary Embodiment 1)

[0011]FIG. 1A through FIG. 1D are sectional views of a circuit boardaccording to exemplary embodiment 1 of the present invention. Insulatingsubstrate 101 may be a glass-epoxy board made of glass cloth impregnatedand/or coated with epoxy resin, a resin board made of woven cloth orunwoven cloth of resin fibers, such as aramid impregnated with resinsuch as epoxy, or a film board made of a plastic film such as polyimidecoated with resin such as adhesive material. Insulating substrate 101 isprovided with through-hole (via-hole) 102, as shown in FIG. 1A, by laserbeam using carbon dioxide, YAG, and the like, or drilling. Via-hole 102is more favorably formed by laser machining than mechanical method usinga drill, since the hole can have its small diameter and its smoothperipheral edge as if it is melted, which is effective for a filling ofconductive material at a subsequent process.

[0012] Via-hole 102 formed in the insulating substrate 101 is thenfilled with conductive material 103, as shown in FIG. 1B. Conductivematerial 103 may employ pasty material containing a granular metal in aform of fine spherical or scaly particles of approximately 0.1 μm to 50μm mixed with organic solvent and/or resin. The metal may be copper,alloy of copper and silver, or copper having its surface coated oralloyed with another metal like silver, gold or the like. Copper canmake particles of spherical shape inexpensively. Copper may be mixedwith or coated on its surface with other metal of lower hardness andlower electric resistance than those of copper, such as silver, gold, orthe like. The silver or gold may be transformed by press-working in asubsequent process. Accordingly, such metal of lower resistance canfurther reduce contact resistance since the particles have increasedcontact surface areas on the copper. Conductive material 103 may befilled into the via-hole 102 by such a method as printing conductivematerial 103 on one side of the via-hole 102 while the other side is,for example, vacuum-suctioned.

[0013] Then, conductive layers 104, for instance, copper foils areplaced on respective sides of insulating substrate 101 having via-hole102 filled with conductive material 103, as shown in FIG. 1C. Metal 105which can be alloyed with copper is deposited on conductive layer 104deposited at least at an area contacting with conductive material 103.Metal 105 has a lower melting point than copper, and is alloyed withcopper by a temperature-independent reaction such as adhesion andpressure bonding between the metals by a pressure applied in thepress-working, and/or by a thermal energy given by heat-press that heatsthem while applying a pressure in the process, which will be describedlater. Metal 105 is bonded to each of conductive layers 104 bydepositing metal of comparatively low melting point, such as tin, zinc,silver, palladium, indium, or bismuth, by a method, such as plating,thermal spraying. In this embodiment, metal 105, if having a granularform, is alloyed easily since contact areas among the granules becomesmaller, thereby increasing energy of reaction in pressure andtemperature that they receive per unit area. Conductive layers 104placed on both sides of insulating substrate 101 are then pressedexternally while being heated at least at a pressure and a temperaturethat produce adhesion between insulating substrate 101 and conductivelayers 104. The temperature needs to form alloy of the metal to bealloyed with copper. The temperature is preferably 120° C. or higher,but 300° C. or lower, and more preferably ranges from 200° C. to 270° C.In addition, the higher the pressure applied in this process, the betterit results. The pressure does not excessively compress insulating resin101, and thus is, for example, 200 kg/cm² or less. Indium and bismuthhave melting points of 157° C. and 271° C., respectively, as low as tin.They start reactions, e.g. of alloying, at a temperatures ranginggenerally from 60 to 70% of their melting points. The reactions areaccelerated further in this process if they receive additional energyof, e.g., pressure or mechanical activity. Alloy layers containing tinhaving a melting point of 232° C., while copper has that of 1084° C. ofcopper, has a desirable condition for an electric resistance andmechanical strength. Tin can be alloyed only with a portion approx. 10%or less of the entire copper. Furthermore, zinc, silver, and palladiumhaving respective melting points of 419° C., 962° C., and 1554° C. arecapable of decreasing the contact resistance substantially since theyform diffusion layers, and/or since they produce adhesion or pressurebonding even though they do not melt to be alloyed.

[0014] Reaction layers 106 are formed as shown in FIG. 1D by adhesion,pressure bonding, and/or alloying at connecting portions at boundariesbetween conductive layers 104 and the conductive material 103 filled inthe via-hole. Copper and metal 105 to be alloyed with the copper to formthe reaction layers 106 produce diffusion layers and/or alloy layersonly on copper surfaces with practically no need to melt the copper.This increases mechanical strength and decreases electric resistance atthe connecting portions. In addition, the copper, since keeping itsintrinsic low resistance, provides the connecting portions having a lowresistance and high mechanical strength.

[0015] (Exemplary Embodiment 2)

[0016]FIG. 2A through FIG. 2D are sectional views of a circuit boardaccording to exemplary embodiment 2 of the present invention.

[0017] Insulating substrate 201 made of, for example, a glass epoxyboard, a resin board, or a film board is provided with via-hole 202, asshown in FIG. 2A, formed therein in the same manner as described inembodiment 1. Then, the via-hole 202 formed in insulating substrate 201is filled with conductive material 203 not containing copper butconsisting of metal that forms alloy with copper, as shown in FIG. 2B.Conductive material 203 can be filled into via-hole 202 by a method,such as printing conductive material 203 on one side of via-hole 202while the other side is vacuum-suctioned. Conductive material 203 has alower melting point than copper, and is alloyed with copper by atemperature-independent reaction, such as adhesion or pressure bondingbetween these metals by a pressure applied in the press-working, and/orby a thermal energy given by heat-press that heats them while applying apressure in the process described later. Conductive material 203 maysuitably employs soft metal, such as tin, zinc, silver, palladium,conductive indium, bismuth, that have comparatively low melting pointsand low hardnesses. The metals may be used as material of lead freesolder that does not contain noxious lead. Conductive material 203 maybe suitably formed in a pasty form made of granular metal in a form offine spherical or scaly particles of approximately 0.1 μm to 50 μm mixedwith organic solvent and/or resin. The metal has relatively high meltingpoint and high hardness, and does not contain copper. The metal may beany other metal excluding copper having surface coated withaforementioned metal, alloy of the aforementioned metal, or any otherpure metal.

[0018] Next, conductive layers 204 made of copper foils or the like areplaced on both sides of the insulating substrate 201 which has via-hole202 filled with conductive material 203, as shown in FIG. 2C. Conductivelayers 204 are then pressed from the outside while being heated at leastat a pressure and temperature that produce adhesion between insulatingsubstrate 201 and conductive layers 204. The temperature may bepreferably 120° C. or higher, but 300° C. or lower, which can induce areaction of forming alloy between conductive material 203 and conductivelayers 204. The temperature may more preferably be between 200° C. and270° C. In addition, the higher the pressure applied in this process thebetter it results. The pressure does not excessively compress theinsulating substrate 201, and thus is, for example, 200 kg/cm² or less.Conductive material 203 can be alloyed easily because of its granularform having a smaller contact area than the granules, thereby increasingenergy of reaction in pressure and temperature that they receive perunit area. In addition, conductive material 203, since being filled invia-hole 202 contains soft metal of low hardness, increases contactareas by deformation resulting from a pressure of press working, therebydecreasing an electric resistance in via-hole 202. Nickel, chromium,molybdenum, and tungsten, which have high hardness and high meltingpoint, are effective for a core material to be coated. They help thepressure to act more effectively upon conductive material 203 in thepress-working since conductive material 203 consists of soft metalhaving a comparatively low hardness. In other words, they not onlypromote conductive material 203 to be alloyed in itself, but alsoincrease contact area that can reduce the contact resistance by thepress-working.

[0019] Reaction layers 206 are formed as shown in FIG. 2D by thepress-working which produces adhesion, pressure bonding, and/or metalalloy of copper and the metal to be alloyed with copper, as discussedabove, at connecting portions at boundaries between conductive layers204 and conductive material 203 filled in the via-hole. It ispractically unnecessary to melt the copper in reaction layers 206 if thecopper and conductive material 203 to be alloyed with the copper areused. This is because reaction layers 203 need to be formed by diffusionand/or alloying only on surfaces of the copper. This can thus increasemechanical strength of the connecting portions, and decrease an electricresistance. In addition, the copper, since keeping its intrinsic lowresistance, provides the connecting portions having a low resistance andhigh mechanical strength. Reaction layers 206 start reactions to form,for example, alloy at a temperature generally 60 to 70% of its meltingpoint similarly to embodiment 1. The reactions are accelerated evenfurther if they receive additional energy of, for example, pressure ormechanical activity. The alloy layers is in a desirable condition for anelectric resistance and mechanical strength if tin is alloyed only witha portion approx. 10% or less of the entire copper. Moreover, contactresistances of the layers can be further decreased since they formdiffusion layers, and/or since they produce adhesion or pressure bondingeven if they are not alloyed.

[0020] (Exemplary Embodiment 3)

[0021]FIG. 3A through FIG. 3D are sectional views of a circuit board,illustrating a method of manufacturing the board according to exemplaryembodiment 3 of the present invention.

[0022] As shown in FIG. 3A, insulating substrate 301 made of, forexample, a glass epoxy board, a resin board, or a film board is providedwith via-hole 302 formed therein similarly to embodiments 1 and 2. Then,via-hole 302 is filled with conductive material 303 containing copperand another metal that can be alloyed with copper, as shown in FIG. 3B.Conductive material 303 can be filled into via-hole 302 by a method,such as printing conductive material 303 on one side of via-hole 302while the other side is vacuum-suctioned. The metal contained inconductive material 303 to be alloyed with copper has a lower meltingpoint than copper, and is alloyed with copper by atemperature-independent reaction, such as adhesion or pressure bondingbetween these metals by a pressure applied in the press-working, and/orby a thermal energy given by heat-press that heats them while applying apressure in the process, which will be described later. The metal may besuitably employ tin, zinc, silver, palladium, conductive indium, orbismuth, which is soft metal having comparatively low melting point andlow hardness. Material suitable for conductive material 303 is in pastyform made of granular metal in a form of fine spherical or scalyparticles of approximately 0.1 μm to 50 μm mixed with organic solventand/or resin. The metal may be copper or any other metal of relativelyhigh melting point and high hardness with a surface coated with theabove-mentioned metal, alloy of copper with the above-mentioned metal,or any other pure metal.

[0023] Next, conductive layers 304 consisting of copper foils or thelike are placed on both sides of insulating substrate 301 havingvia-hole 302 filled with conductive material 303, as shown in FIG. 3C.Conductive layers 304 are then pressed from the outside while beingheated at least at a pressure and a temperature that produce adhesionbetween insulating substrate 301 and conductive layers 304. Thetemperature may be 120° C. or higher, but 300° C. or lower, which makesthe copper in conductive layers 304 to form alloy with the metalcontained in conductive material 303 and the copper in conductivematerial 303 to be alloyed with the afore-mentioned metal. Thetemperature is more preferably between 200° C. and 270° C. In addition,the higher the pressure applied in this process the better it results.The pressure, however, does not excessively compress insulatingsubstrate 301, and thus is preferably 200 kg/cm² or less. Conductivematerial 303 can be alloyed easily because of its granular form thathaving a smaller contact area among the granules, thereby increasingenergy of reaction in pressure and temperature that they receive perunit area. In addition, since conductive material 303 filled in via-hole302, since being composed of soft metal of low hardness, has an contactarea increased by deformation resulting from a pressure of pressworking, thereby decreasing an electric resistance in via-hole 302.Copper is suitable as core material to be coated for conductive material303 since being relatively inexpensive, having a low electricresistance, and easily forming fine particles of spherical shape. Evenwith any such metal having relatively high melting point and highhardness, the pressure acts more effectively in the press working uponconductive material 303 since the metal also contains the soft metalhaving comparatively low hardness. This not only promotes conductivematerial 303 to be alloyed in itself, but also increases the contactarea and reduces the contact resistance by the press working and by thelow electric resistance.

[0024] Copper is contained in conductive material 303 preferably at 50%or less for the electric resistance, and more preferably at 10% or lessfor both the electric resistance and mechanical strength. Moreover,contact resistances between conductive material 303 and conductivelayers 304 can be further decreased since they form diffusion layers,and/or since they produce adhesion or pressure bonding even if they arenot alloyed.

[0025] Reaction layer 306 is formed as shown in FIG. 3D by thepress-working which produces adhesion, pressure bonding, and/or alloy ofcopper and the metal to be alloyed with the copper, as discussed above,at connecting portions at boundaries between conductive layers 304 andconductive material 303, as well as an interior of via-hole 302. Thus,the copper need not melt in reaction layer 306 containing the copper andafore-mentioned metal. The copper forms diffusion layers or alloy layersonly on surfaces of the copper. This improves mechanical strength anddecreases electric resistance of reaction layer 306, and thus providesconnecting portions with low resistance and high mechanical strengthsince interiors of individual copper particles contained in reactionlayer 306 keeps its intrinsic low resistance. Reaction layer 306 startsreaction, for example, to form alloy at a temperature generally of 60 to70% of the melting point similarly to embodiment 1. The reaction isaccelerated even further if it receives additional energy of, forexample, pressure or mechanical activity.

[0026] Reaction layers are formed by adhesion, pressure bonding, and/oralloying at connecting portions in the boundaries between the conductivelayers and the conductive material filled in the through via hole.Accordingly, the circuit board is provided with a low resistance andhigh mechanical strength at the connecting portions since the reactionlayers have high mechanical strength and low electric resistance whileinterior of the copper contained therein keeps its intrinsic lowresistance. In addition, a resistance of the conductive layers on bothsides of the insulating substrate at the via-hole is reduced since aninterior of the reaction layer reduces the resistance of the via-hole.Furthermore, mechanical strength of the connecting portions increases,and the reliability improves by mechanically securing the connections.Moreover, since the via-hole is filled with the conductive material, acomponent can be mounted on a surface of the conductive layer includingan area above the via-hole, which improves downsizing of the circuitboard as well as wiring flexibility.

[0027] The present invention provides the same effect for a circuitboard having a conductive layer formed previously on only one side of avia-hole, i.e., a blind via-hole. In addition, a similar advantage isobtainable with a multi-layered board fabricated by repeating theprocess described in any of the foregoing embodiments. Like advantage isalso accomplished with a circuit board having a conductive layer madepreviously by transferring a pattern-formed metal foil.

[0028] In the foregoing exemplary embodiments, the conductive layers arecopper foils, and the conductive material contains copper and anothermetal that is alloyed with copper. The conductive layer may be composedof other conductive substance instead of copper, and the conductivematerial may contain any material that can be alloyed with the substancein order to achieve like advantages.

INDUSTRIAL APPLICABILITY

[0029] In the circuit board of the present invention, a conductivematerial filled in a via-hole formed in an insulating substrate and aconductive layers on both sides of the insulating substrate are securelyconnected electrically as well as mechanically with high reliability, byforming metal alloy of a part of metals that compose the layers and thematerial.

1. A circuit board comprising: an insulating substrate having a via-holeformed therein; a conductive material filled in said via-hole;conductive layers on both sides of said insulating substrate,respectively; first alloy containing component material of saidconductive material and component material of said conductive layers;and second alloy contained in said conductive material.
 2. The circuitboard according to claim 1, wherein at least one of said conductivelayers and said conductive material contains copper, and wherein saidfirst alloy contains copper.
 3. The circuit board according to claim 1,wherein said conductive layers contain copper, and wherein said firstalloy contain said copper in said conductive layers.
 4. The circuitboard according to claim 1, wherein said conductive material containscopper, and wherein said first alloy contains said copper in saidconductive material.
 5. The circuit board according to claim 4, whereinsaid conductive material contains 50 wt % or less of copper.
 6. Acircuit board comprising: an insulating substrate having a via-holeformed therein; a conductive material filled in said via-hole, saidconductive material containing alloy; and conductive layers on bothsides of said insulating substrate, respectively.
 7. A method ofmanufacturing a circuit board comprising the steps of: filling avia-hole formed in an insulating substrate with a conductive material;disposing conductive layers on both sides of the insulating substrate,respectively; and forming alloy of component material of the conductivematerial with component material of the conductive layers.
 8. The methodaccording to claim 7, wherein the conductive layers contain copper,wherein the component material of the conductive material is metal, andwherein the conductive material includes a particle containing themetal.
 9. The method according to claim 8, wherein the metal has a lowermelting point than copper.
 10. The method according to claim 7, whereinthe conductive material contains copper, and wherein the conductivelayers each includes a particle containing metal to be alloyed withcopper placed on a surface of the conductive layers.
 11. The methodaccording to claim 10, wherein the metal has a lower melting point thancopper.
 12. The method according to claim 7, wherein the conductivematerial contains copper, wherein the conductive material containscopper and metal which can be alloyed with copper, and wherein said stepof forming the alloy of the component material of the conductivematerial with the component material of the conductive layers comprisesthe step of forming the alloy containing at least a part of the copper.13. The method according to claim 12, wherein the metal has a lowermelting point than copper.
 14. The method according to claim 7, whereinsaid step of disposing the conductive layers comprises the step ofpressing the conductive layers onto the insulating substrate, andwherein said step of forming the alloy of the component material of theconductive material with the component material of the conductive layerscomprises the step of compressing the conductive material simultaneouslyto said step of pressing the conductive layers.
 15. The method accordingto claim 7, wherein said step of disposing the conductive layerscomprises the step of heat-pressing the conductive layers onto theinsulating substrate, and wherein said step of forming the alloy of thecomponent material of the conductive material with the componentmaterial of the conductive layers comprises the step of compressing andheating the conductive material simultaneously to said step ofheat-pressing the conductive layers.
 16. The method according to claim15, wherein the component material of the conductive layers containscopper, and wherein the component material of the conductive materialcontains metal having a melting point lower than a temperature in saidstep of heat-pressing the conductive layers.
 17. The method according toclaim 15, wherein the component material of the conductive layerscontains copper, and wherein the component material of the conductivematerial contains one of tin, zinc, silver, palladium, indium, andbismuth.